LATERAL/DIRECTIONAL STATIC STABILITY

When addressing the question of lateral and directional stability it is convenient to assume that the motion can be analyzed separately from the longitudinal motion.

Cross-coupling terms are therefore ignored to obtain a reasonable feel for the subject but it must be remembered that in the real case cross-coupling will occur and these effects will almost certainly modify the results obtained. Here only the lateral/ directional static stability of the helicopter shall be considered which is dominated by the derivatives Lv, Lp, Nv, and N. Major contributions to these derivatives come from the main rotor, the tail rotor, the fin and the fuselage. The control displacements, required to maintain steady side-slipping flight at constant heading which are related to the corresponding static stabilities, are also described. The lateral cyclic displacement is related to the lateral (rolling) static stability, (Lv) and the yaw pedal displacement is related to the directional (yawing) static stability, (Nv). In order that the control movements should be in the conventional sense it is necessary that both Lv and Nv are stabilizing. It is worth remembering the sign convention for control deflections; positive control displacement produces negative aircraft response (lateral cyclic stick movements to the left and left push of the yaw pedals are taken as positive).

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